Lithium ion batteries (LIBs) are widely used in consumer electronics, such as laptop computers, camcorders, cameras, and cell phones, and are now being considered for applications in electric vehicles. With pressing worldwide environmental concerns, lithium ion batteries have been actively proposed for applications in electric vehicle (EV), hybrid electric vehicle (HEV) and plug-in hybrid-electric vehicles (PHEVs). However, their use has been largely obviated due to safety concerns related to the volatile liquid organic electrolytes used in these batteries. The volatile electrolytes are prone to local overheating, with the possibility of fire or explosion, particularly in the event of a short circuit or structural damage. Ionic liquids, which are known for their non-flammability, high electrochemical stability and negligible vapor pressure, are good candidates for tackling the safety issues in lithium ion batteries.
Much work has been directed to testing ionic liquids as safer electrolytes for lithium ion batteries. As a routinely studied example of ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI.TFSI) has the advantages of low viscosity and high ionic conductivity. However, EMI.TFSI has a very poor electrochemical window, particularly its relative high reduction potential of 1.0V versus Li/Li+, which is too positive to allow lithium deposition or intercalation. For this reason, EMI.TFSI is not suitable for lithium ion batteries. Ionic liquids containing saturated quaternary ammoniums are more resistant toward oxidation and reduction than the imidazolium cation, and thus, they generally have much larger electrochemical window than the corresponding imidazolium compounds. However, they and other ionic liquids have relatively poor capability in forming an efficient solid electrolyte interphase (SEI), which results in continual reaction and losing capacity with cycling.
Graphite has been routinely used as standard anode material in commercial LIBs because of its low cost, low lithium intercalation potential, and good cycling stability. To be functional in LIBs, the graphite electrode surface has to be passivated via electrolyte decomposition to form a good thin layer of solid electrolyte interphase (SEI) that prevents continual electrolyte decomposition during the following cycles. For the sole SEI purpose, ethylene carbonate (EC) has become an indispensable component in the mixed carbonate electrolytes used in commercial LIBs. Unfortunately, EC has a high melting point of 37° C., which limits the low-temperature performance of LIBs, even with a combination of linear carbonates. By contrast, propylene carbonate (PC), with a similar cyclic structure as EC, has a much lower melting point of −49° C., and thus, offers many advantages over the carbonate mixtures, particularly with regard to low-temperature performance. However, PC by itself is not compatible with the graphite electrode since it is known to co-intercalate with lithium ions into graphene layers, thereby causing exfoliation of the graphite. In an effort to improve the compatibility of PC with graphite electrode, additives such as vinylene carbonate, vinyl ethylene carbonate (VEC), vinyl ethylene sulfite (VES), butylene carbonate (BC), 2-phenylimidazole, and others, have been incorporated into the electrolyte.
On the cathode side of LIBs, either pure or doped LiCoO2 (or LiMnO4) are used in the commercial cells. Recently, with an effort to increase the energy density of the LIBs, 5.0V cathodes, such as LiNi0.5Mn1.5O4, LiNixCo1-xPO4, and LiCuxMn2-xO4, have been successfully developed (Cresce, A. V., et al., Journal of the Electrochemical Society, 2011, 158, A337-A342). However, the electrolytes used for these 5.0V cathodes are still based on carbonate mixtures, which have electrochemical stability window below 4.5V. In these mismatched cells the SEI layer on the 5.0V cathode surface is relied upon to stop the continual electrolyte decomposition to make the battery function. Unavoidably, the battery usually exhibits poor cycling stability and high polarization due to the high resistive SEI layer formed on the cathode surface. On the other hand, there are two kinds of electrolytes, namely alkyl sulfones and ionic liquids, which exhibit high oxidative stability (well above 5.5V versus Li/Li+) to match that of 5.0V cathode. However, unfortunately, these electrolytes suffer from high viscosity and poor compatibility with graphite electrode.